Helical line scanners are known in the art. U.S. Pat. Nos. 3,523,160 and 4,494,821 describe optical scanning devices utilizing a flat surface helicoid reflector formed on a rotatable cylindrical drum. Such devices scan a line of a scanning medium by reflecting a light beam from the surface of the rotating helicoid reflector.
The helicoid reflector has advantages over existing scanners. As discussed in the article by Leo Beiser, "Generalized Equations for the Resolution of Laser Scanners", Applied Optics, Oct. 15, 1983, Vol. 22, No. 20, pp. 3149-3150, existing scanners use rotating pyramidal or polygonal mirrors inherently have format and resolution limitations. The helicoid reflectors have none of these limitations.
A single helicoid reflector is less prone to production errors thereby providing a scan without noticeable artifacts. However, by its nature, a flat helical surface cannot be completely flat. In fact, it inherently has a shape defined by a saddle function. Thus, a collimated light beam reflecting from such a surface becomes diverged.
The optical scanning device of U.S. Pat. No. 3,523,160 uses a lens after the helical surface to refocus the beam. The optical scanning device of U.S. Pat. No. 4,494,821 improves on the device of U.S. Pat. No. 3,523,160 so as to reduce the divergence of the light beam through the introduction of a narrow slit. The narrow slit is not advantageous since it produces a relatively large spot in the shape of a parallelogram.
Unfortunately, the prior art helicoid reflectors by themselves do not compensate for their existing aberrations. They require use of additional optical elements, such as the narrow slit of U.S. Pat. Nos. 4,494,821 or the lens of 3,523,160.
In order to provide a beam which is perpendicular to a scanning medium, a reflecting surface tilted to the axis of helix is desired. This angle is called the pitch angle.
As is known in the art, all helices have a pitch angle, being the angle the helical surface makes with the axis of the helix. The pitch angle is a function of the length L of the helix and its diameter D, as follows: EQU tan .alpha.=.pi.D/L (1)
the longer the desired scan length, the larger the required diameter, where, for a 45.degree. pitch angle, such as is utilized in the devices mentioned hereinabove, the diameter must be generally a third of the length. Thus, the physical construction of long scanning versions of the abovementioned devices becomes cumbersome.